you need to determine if you were a victim a scam which i strongly suspect. in such case you have a casualty loss which is no longer deductible in 2018 if you could establish that in 2017 you knew the website was gone and you could sell the crypto due to a scam you could file an amended 2017 return. It still would be a casualty loss, form 4684. casualty losses go on schedule A - itemized deductions.
